Thin ceramic coatings

ABSTRACT

The invention relates to a method for applying a non-permeable, ceramic layer of a thickness of not more than about 100 μm to a ceramic or metallic body by applying a solution of one or more elastomers, which elastomers contain substantially not exclusively sulfur, carbon, hydrogen and oxygen, optionally drying, and pyrolyzing to form a porous layer and subsequently sintering at increased temperature to form a non-permeable ceramic layer.

This invention relates to thin ceramic layers applied to non-porous orcoarse-porous ceramic or metallic substrates.

Non-porous ceramic layers are often applied as glazing to ceramicsubstrates or as enamel to metallic substrates. The aim of applying thelayers is generally protection or embellishment.

The present invention is directed especially to the use of thin ceramiclayers in the protection of metals and alloys. Primarily, this involvesscreening against the attack of the metal or the alloy by reaction withcarbon-containing molecules in gas form. It has been observed that uponexposure of metal and alloy surfaces to carbon-containing gas molecules,such as methane or higher hydrocarbons or a mixture of carbon monoxideand hydrogen, metal or alloy particles disappear from the surface. As aresult, the thickness of the metal or the alloy can decrease rapidly,giving rise to fracture in equipment working under increased pressure.In cases where work is not done under pressure, the loss of metal or thealloy can cause leakage. In other cases, the exposure of metals oralloys to hydrocarbons at increased temperature leads to the depositionof a relatively dense layer of carbon on the metal or alloy surface.This can give rise to clogging and is therefore undesirable. Beforeclogging occurs, however, the heat transfer from the metal or alloy wallto a gas stream is found to decrease strongly.

In many technically important cases, such as, for instance, in naphthacracking plants, a significant reduction of the heat transfer isunallowable, since it results in a strong decrease of the capacity ofthe plant. The plant must be stopped and the carbon layer must beremoved, for instance by oxidation. In general, this occurs by reactionwith oxygen or with steam.

Technically, it is of great importance to protect metal or alloysurfaces against loss of metal or alloy particles, or against thedeposition of carbon layers, by the use of a suitable coat. According tothe present state of the art, the application of such a protective layerhas been found not to be properly possible. It has been attempted, bystarting from an aluminum-containing alloy, to apply a protectivealuminum oxide layer to the metal surface. An example of such an alloyis Fecralloy®. In practice, however, a layer formed in such a way wasfound not to protect the metal surface sufficiently. An additionaldrawback of Fecralloy® is that this alloy, like otheraluminum-containing alloys, cannot be welded.

In general, the fact that aluminum-containing alloys cannot be welded isa drawback of alloys that are resistant to oxidizing gases at highlyincreased temperature. A second object of the present invention istherefore the provision of non-permeable, oxidation-resistant ceramiclayers on metals of good weldability. In that case, the metals or alloyscan first be brought into the desired form by welding, whereafter theprotective layer is applied.

In this connection, the invention is directed especially to renderingmetal gauzes resistant. In catalytic reactions at (strongly) increasedtemperature, because of the intrinsic high reaction rate, no largecatalytically active surface area per unit of volume is necessary. Thesurface area of a metal gauze is sufficient. In the oxidation of ammoniato nitrogen oxide in the production of nitric acid, use is thereforemade of platinum or palladium gauzes, to which often slight amounts ofother precious metals have been added. A major drawback is that theprecious metal disintegrates during the catalytic reaction. Initially,use was made of a gauze of gold, arranged under the platinum gauze tocatch the platinum particles. Later, a platinum gauze was used to catchthe small platinum particles formed. In that case, no platinum-goldseparation is needed to recover the platinum. If, in fact, no catalyticreaction proceeds over the platinum, the platinum does not disintegrate.To increase the productivity of nitric acid factories, it is highlyattractive to work with pure oxygen instead of air; also at increasedoxygen pressure the productivity increases strongly. However, because ofthe greatly accelerated disintegration of the precious metal gauze athigher oxygen pressure, this has not been found possible so far.Applying the precious metal in finely divided form to a stable gauzewould enable an important improvement of the nitric acid process. Thisprocess has not been fundamentally improved since the invention byOstwald at the end of the nineteenth century.

In the Andrussow process, in which, at temperatures above about 1000°C., ammonia is allowed to react with methane to hydrogen and hydrogencyanide, also a noble-metal gauze is used. In this case too, a stablermetal gauze is of great significance. Finally, processes are currentlybeing worked on, to produce synthesis gas, a mixture of carbon monoxideand hydrogen, by contacting a stream of methane and pure oxygen with acatalyst, for instance platinum, at temperatures above 1000° C. Suchprocesses too could highly advantageously utilize precious metalsapplied in finely divided form to stabilized metal gauze.

Obviously, according to the state of the art of enameling, much researchhas already been done on the application of protective ceramic coatingsto metal and alloy surfaces. In general, the conditions and the chemicalcomposition necessary to accomplish a good bonding to the metal areknown. However, it has been found to be very cumbersome to apply anenamel having a softening point or melting point that lies at a hightemperature with a homogeneous chemical composition as a thin uniformlayer to metal or alloy surfaces. According to the present state of theart, it is also cumbersome to accurately set the chemical composition ofthe protective ceramic layer. This is an important objective of theinvention.

In the use of porous ceramic layers on solid surfaces, an object can bethe protection against too high a temperature of the metal or the alloyupon exposure to a high-temperature gas stream. Considered in particularin this connection are gas turbines, where the metal or the alloyexhibits too slight a mechanical strength at the desired hightemperatures. In that case, use can be made of a porous layer of athermostable material which, through an effectively low heatconductivity of the porous layer, leads to a temperature profile overthe porous layer such that the temperature of the metal or the alloydoes not exceed a particular limit value. Firm anchorage of such aporous layer, when used in gas turbines, is obviously an importantcondition.

A second use contemplated by the invention is the use of a porous layerapplied to a solid surface as catalyst. According to the prior art, suchlayers are applied to solid surfaces by using so-called dip coatingtechniques. The surface to be covered is immersed in a suspension of thecatalytically active material and the surface is removed from thesuspension at an empirically determined speed. Depending on theviscosity and the other properties of the suspension, a layer of thecatalytically active material of a certain thickness then deposits onthe substrate. For the production of exhaust gas catalysts, this methodis presently used on a large scale. Used as substrates are, virtuallyexclusively, ceramic monoliths. To date, however, no successful attemptshave been made to modify the dipcoat or washcoat process such thatfirmly anchored catalytically active layers can be applied to metalsurfaces.

According to the prior art, a high-porous layer exhibiting betterbonding can be applied to ceramic and metallic surfaces by starting fromsolutions of silicone rubber or the titanium-containing equivalentthereof. This is described, for instance, in U.S. Pat. No. 5,472,927. Bydipping or by spin-coating, a thin layer of such an elastomer can beapplied to the surface to be covered. Pyrolysis of the thin layer of theelastomer resulting after drying then leads to a high-porous layer of aceramic material. A so prepared layer of silicon dioxide maintains theporosity up to temperatures of about 700° C. The thermal stability ofthe ceramic layer, as well as the pore distribution of the layer, can beset by adding, for instance, aluminum compounds to the solution ofsilicone rubber. A compound suitable for this purpose is, for instance,aluminum sec-butoxide. Mixtures of silicone rubber and thetitanium-containing equivalent can be used to apply silicon dioxide withan adjustable amount of titanium dioxide.

In general, the thus obtained ceramic layers contain no catalyticallyactive components. According to the present state of the art, those areprovided by impregnation of the porous ceramic layer with a solution ofa precursor of the catalytically active material. Through a thermaltreatment, the precursor can be converted to the desired catalyticallyactive component.

The present state of the art also encompasses wholly or partlyconverting a porous silicon dioxide layer applied to a solid substrateto a synthetic clay mineral, as is described in WO-A-96/07613. Clayminerals are catalytically of interest as solid acid catalysts.

The application of catalytically active materials to solid, non-porousor little porous surfaces has been found to be of great value when usedin gas streams where a low pressure drop is essential. As mentionedabove, in many such cases monoliths are used. Also other materials witha low pressure drop have been developed, whereby an intensive contactbetween a gas stream and a catalyst surface is effected. Examples aresintered metals, ceramic and metallic foams and in particular specialreactor packings of specially shaped metal foils. The operation ofspecial reactor packings has been described by G. Gaiser and V. Kottkein Chem.-Ing.-Technik 61 (1989) no. 9, pp. 729-731. With all thesematerials, it is a requirement that the catalytically active materialcan be applied to the surface of the structure of the reactor packing soas to be firmly anchored thereto.

Finally, reference can be made to a few other areas where catalyticallyactive materials applied to solid surfaces can be of great significance.These areas cover catalytic liquid-phase reactions or catalyticreactions where a gaseous and a liquid reactant play a role, such ascatalytic hydrogenation or oxidation reactions. According to the presentstate of the art, in these reactions, use is made of suspended catalystsor of a fixed catalyst bed through which the reactants are passed. Wellknown is the use of a fixed catalyst bed through which a liquid reactanttogether with a gas stream is allowed to flow down, a so-called trickleflow process. In a fixed catalyst bed, catalyst bodies with dimensionsof at least a few millimeters must be used, because otherwise thepressure drop becomes too high. As a consequence, because of the lowdiffusion coefficient in liquids, in a fixed catalyst bed only the outeredge of the catalyst bodies contributes effectively to the catalyticreaction. This implies poor utilization of the catalyst and may alsohave a highly adverse effect on the selectivity of the catalyticreaction. In suspended catalyst bodies, much smaller bodies can be used,for instance of dimensions of 3 to 100 mm. Now the utilization of thecatalyst is much better and the selectivity is not affected. When usingsuch small catalyst bodies, however, the separation of the catalyst fromthe reactor product through settlement and decanting, filtration orcentrifugation is laborious. Also, the catalyst bodies are often subjectto wear, so that extremely small catalyst particles cannot be separatedfrom the product and the reaction product is contaminated.

When applying the catalyst as a thin layer on a solid surface, theadvantages of the fixed catalyst bed, no separate separation of thecatalyst, are combined with those of a suspended catalyst, efficientutilization of the catalyst and good selectivity. Also, a suitable flowpattern of the liquid, and possibly the gas, around the catalyst can berealized. Thus it is possible first to mix the reactants very intimatelybefore they contact the catalyst.

Surprisingly, it has been found that a porous ceramic layer applied to aceramic or metallic substrate, upon treatment at a sufficiently hightemperature, for instance by sintering, preferably at a highertemperature, can be converted to a dense, non-permeable layer. Accordingto the known state of the art, the porous layer is applied by pyrolysisof a suitable polymer, or a polymer mixture or polymer solution.

According to the invention, the porous layer can be applied tonon-porous or coarse-porous ceramic or metallic substrates.‘Coarse-porous’ in this context is understood to mean ‘containing poresof a diameter of about 1 mm or more’. The invention encompasses bothnon-porous, and hence non-permeable, ceramic layers and (high-)porousceramic layers.

Suitable polymers are polymers based on organometallic compounds, suchas compounds of titanium, zirconium or aluminum. In the context of theinvention, silicon compounds are also regarded as organometalliccompounds. In general, as polymer, elastomers can be used. In thecontext of the invention, elastomers are defined as polymers having aglass transition temperature of less than 0° C. Eminently suitablepolymers are rubbers that are pyrolyzable to ceramic material. It hasbeen found that silicone rubber is an eminently suitable polymer.

The thickness of the layer can be varied within wide limits. Ifnecessary, if relatively thick layers are desired, several layers areapplied one after another. It has been found to be advantageous topyrolyze the elastomer layer before a new layer is applied. Preferably,thin layers are applied, ‘thin’ being understood to mean a thickness ofless than 1 mm to a thickness of about 100 mm. With thin layers, thedifference in thermal expansion between the ceramic layer and thesubstrate plays a comparatively minor role.

The chemical composition of the ceramic layer can be adjusted asdesired. First of all, this is possible by setting the composition ofthe solution of the polymer, in accordance with the state of the art, byadding to, for instance, silicon dioxide, certain components, such asaluminum or titanium. The addition of titanium compounds readily leadsto acid-resistant enamel layers, which are part of the invention.According to the method of the invention, such layers can easily andrapidly be applied to the wall of reactors. Also, by incorporation ofcontrolled amounts of aluminum and/or titanium ions, the melting orsoftening point of the material of the layer can be controlled.

Alkali-resistant layers are obtained according to the invention byadding zirconium to silicon dioxide, alone or in combination with tinoxide. According to the invention, boron oxide is added preferably inthe form of suitable boron compounds, such as for instance aluminumborohydride, to the solution of the elastomer.

A second procedure for incorporating certain components into the initialporous layer is impregnation of the porous layer with solutions ofsuitable compounds or deposition-precipitation of certain compounds inthe porous layer. These procedures are attractive especially for theapplication of components such as nickel oxide and cobalt oxide.Reaction with the silicon oxide can be readily obtained bydeposition-precipitation of these elements, but impregnation is alsoattractive in many cases. It is known that nickel oxide and cobalt oxidegreatly improve the bonding of silicon dioxide-containing layers tometal and alloy surfaces. According to the invention, the impregnationof a suitable solution of components to be included in the ceramic layeris preferably done in the evacuated layer, whereby a volume of solutionis impregnated which corresponds to the pore volume of the porous layer.

To effect good bonding of the ceramic layer to metal or alloy surfaces,the metal surface must contain a thin oxide layer. When this layer istoo thick, no good bonding is obtained. In traditional enameling, thisis a problem. For use at high temperatures, a ceramic layer having ahigh softening or melting temperature must be applied. When thesoftening or melting point of the enamel is high, the metal or alloysurface, upon application of the layer, is oxidized too strongly. Theresult is then a less good bonding. Surprisingly, it has now been foundthat the pyrolysis of the layer of the elastomer obtained after dryingleads to an exceedingly firmly anchored porous layer. On a metal likealuminum, too, provided that the surface is sufficiently degreased andcleaned, an excellent bonding is obtained. The conversion at highertemperature, following the pyrolysis, to form a non-porous,non-permeable layer, is therefore preferably carried out in an inert gasatmosphere. The fact is, it has been found that transport of oxygenthrough the initially porous ceramic layer at increased temperatures canlead to strong oxidation of the metal at the interface with the ceramiclayer. The metal oxide formed then presses the ceramic layer off themetal or alloy surface. In the pyrolysis of the dried layer of theelastomer, sufficient metal oxide is formed to effect a very goodbonding.

According to a special embodiment of the invention, on a ceramic ormetallic surface, first a protective non-porous, non-permeable layer isapplied, whereafter a porous ceramic layer is deposited on thenon-porous layer. The non-porous layer protects the underlying materialagainst undesired reactions with gases at increased temperature oragainst corrosive action of liquids. This last can lead to a highlyundesirable contamination of the reaction product. The underlyingmaterial can also exhibit undesired catalytic reactions by whichselectivity is impaired. The use of such a non-porous intermediate layerthus constitutes a fundamental improvement of the prior art.

This holds especially for the use of porous layers as catalyticallyactive layers. In fact, catalysts generally lose activity during use,for instance by poisoning. In the case of suspended catalyst bodies,replacing the catalyst is extremely simple. In the case of a packedcatalyst bed, too, the catalyst can be removed from the reactor and bereplaced with a new catalyst charge, although this can be relativelylabor-intensive. If the catalyst is applied as a thin porous layer on aspecial reactor packing, the consequence of deactivation of the catalystmight be that the entire, often costly reactor packing must be replaced.For the use of catalysts according to this invention, it is thereforerequired in many cases that the deactivated catalyst can be relativelyreadily removed from the surface of the reactor packing. According tothe invention, this occurs by treating the reactor packing with analkaline or acid liquid. With most metals, an alkaline liquid can beused because metals such as iron and nickel are resistant to alkalineliquids, while silicon dioxide-containing porous layers often dissolvereadily in alkaline liquids. With a metal such as aluminum, however,this presents problems, since aluminum also dissolves in alkalineliquids, forming hydrogen. Because aluminum, in view of the slightdensity, is especially attractive as reactor packing in larger reactors,protection of the aluminum is highly desirable. Therefore, according toa special embodiment of the invention, the substrate on which thecatalytically active layer is applied, is provided with a non-permeableceramic layer which is resistant to either acid or basic solutions.According to the present state of the art, it is known to manufactureenamel layers that are resistant to acid, to alkali or to both. As notedabove, acid-resistant enamel is obtained by incorporating inter aliatitanium dioxide into the enamel. Resistance to strongly acid liquids isachieved by also incorporating fluorine, which, according to theinvention, is readily possible by impregnation. Alkali-resistant enameltypes often contain zirconium dioxide together with fluorine, which,according to the invention, can also be readily included in the enamel.Also enamel types that are resistant to both acid and lye are knownaccording to the prior art.

For catalytic applications at temperatures above about 700° C., themagnitude of the exposed catalytically active surface is generally ofless importance than the stability of the catalyst system. Therefore,according to the invention, a metal or ceramic covered with anon-porous, non-permeable ceramic layer is used as catalyst support. Thecatalytically active material is applied to this surface in, ifnecessary, finely divided form. In this embodiment, the invention ispreferably practised with gauze-shaped metal substrates.

In certain cases, for instance in catalytic oxidations, it is ofimportance to apply the catalytically active material thermostably in afinely divided form to such a ceramic layer. Surprisingly, it has nowbeen found that a particularly thermostable fine division can beobtained by dissolving a suitable compound of the catalytically activemetal, generally a precious metal, in the solution of the elastomer.Very good results have been obtained with acetic acid salts of palladiumand platinum. As appears from measurements with X-ray photoelectronspectroscopy, a relatively large part of the precious metal, afterpyrolysis and further sintering of the ceramic layer, is present at thesurface.

The present invention can also be used with advantage for theapplication of an oxide layer to a glass surface, for instance with aview to counteracting ‘scuffing’ or preventing the glass surface frombecoming soiled. In this connection, especially titanium oxide andcombinations with titanium oxide are suitable.

The application of zeolite crystals on a solid substrate is of greatpractical significance. The transport in the relatively narrow pores ofzeolites proceeds slowly, so that small crystallites are eminentlysuitable for catalytic reactions. This applies to gas-phase reactions,but especially also to liquid-phase reactions. Crystallites smaller than1 mm cannot be properly separated from the reaction product byfiltration or centrifugation. It is therefore endeavored to synthesizelarger zeolite crystallites, which is often a great problem, orextremely small zeolite crystallites are included in a so-called binder,silicon dioxide or silicon dioxide/aluminum oxide, after which thecombination is formed into larger bodies. Processing the zeolite/bindercombination to form wear-resistant bodies of dimensions of 3 to 10 mm,however, is technically cumbersome, while the binder often impedestransport and can lead to poor selectivity. Zeolite crystallites appliedto a solid substrate are therefore of great technical significance.According to the invention, such ingredients necessary for the zeolitesynthesis as are not already present in the porous layer are impregnatedinto the pores. When, for instance, a template molecule is necessary forthe synthesis of the zeolite, this template in dissolved form isimpregnated in the porous ceramic layer obtained by pyrolysis of thedried elastomer layer. The aluminum necessary for the synthesis of thezeolite will generally be included in the porous layer by dissolving inthe solution of the elastomer. Preferably, the volume of the solution ofthe ingredients of the zeolite synthesis, as described above, is chosento be equal to the pore volume of the porous layer, which is preferablyimpregnated after evacuation. After the impregnation, the layer isbrought under the conditions required for the nucleation and growth ofthe zeolite crystallites. In general, hydrothermal conditions arerequired for this purpose. Especially in the case of metal substrates,it is extremely simple to accurately set and maintain the temperatureduring the synthesis.

It is surprising that in this way zeolite crystallites very stronglybonded to solid surfaces are obtained. It is possible, according to theinvention, to allow the initially porous layer to react wholly or partlyto zeolite crystallites. The thickness of the layer initially applied,consisting substantially of silicon dioxide, determines the density ofthe zeolite crystallites on the surface. It is of great significancethat the surface to be covered with zeolite crystallites does not needto be horizontally oriented during the zeolite synthesis. This makes itpossible without any problem to cover complex reactor packings.

Characteristic of zeolite crystallites that, in accordance with theinvention, are applied to solid ceramic or metallic substrates, is thata metal silicate is present at the boundary layer between the substrateand the zeolite crystallites. It is possible for this layer to comprisenot much more than a few layers of atoms, but the layer is preferablyalways present. As noted above, it is of great importance for thereplacement of deactivated catalysts that the catalytically activelayers or the catalytically active particles can be readily removed,without the ceramic or metallic substrate being affected. Therefore,according to a preferred embodiment of the invention, the zeolitecrystallites are applied to an alkali-resistant non-permeableintermediate layer.

The invention is elucidated with the following examples.

EXAMPLE I

Preparation of Porous Ceramic Layers Based on Silicon Dioxide HavingAdded Thereto Aluminum Oxide, Titanium Dioxide or Zirconium Dioxide.

The starting material was silicone rubber in the form of a commercialproduct, viz. Bison, “transparent”, based on polydimethyl siloxane. Thismaterial was dissolved in diethyl ether. To the obtained solution wasadded aluminum sec-butoxide (ACROS), titanium isopropoxide (JansenChimica), or zirconium isopropoxide (Fluka). The concentration ofsilicone rubber in the solution was 6 to 10% by weight. With aluminum, aseries having different Si/Al ratios was prepared, viz. Si ₉₉Al₁,Si₇₀Al₃₀, Si₈₀Al₂₀, Si₆₅Al₃₅, Si₅₀Al₅₀, and Si₃₅Al₆₅. The titaniumdioxide- and zirconium dioxide-containing silicon dioxide preparationscontained Si/Ti and Si/Zr ratios of 80/20.

After pyrolysis at 873 K, the pore volume of the material was determinedas a function of the Si/Al ratio. While the pure silicon dioxideexhibited a pore volume of about 0.2 ml/g, the pore volume increased to1.4 ml/g at an Al fraction of 0.2, to decrease at higher Al fraction toabout 0.4 ml/g. The accessible surface area, determined by nitrogenadsorption according to the BET theory, increased from 100 m²/g for puresilicon dioxide to 580 m²/g for an Al fraction of 0.75, then to decreaseagain to about 300 m²/g for pure aluminum oxide.

For preparations with Si/Al, Si/Ti and Si/Zr ratios of 80/20, theaccessible surface area was determined as a function of the temperature.The samples were held at the different temperatures for 3 hours. For allthree preparations the surface area of 200 to 260 m²/g after calciningat 873 K gradually decreased to 70 to 180 m²/g after calcining at 1173K. The material with zirconium dioxide was found, after calcining at1173 K, to exhibit the highest surface area. While pure silicon dioxidecan be readily sintered to form a non-permeable layer at about 1073 K,it is necessary, with increasing contents of aluminum, zirconium ortitanium, to heat at considerably higher temperatures. The content ofaluminum, titanium or zirconium is selected depending on the temperatureat which the material covered with a protective layer is to be used.

The material with Si₉₉Al₁ was used to examine the density. To that end,the material was applied to stainless steel. A sample of the stainlesssteel, without having been covered with a layer, was heated at 900° C.in a thermobalance. A rapid weight increase showed that the materialoxidized relatively fast. Analysis showed that the surface was coveredwith a high-porous mass of chromium oxide upon completion of theexperiment. When on a similar plate of stainless steel a layer with thespecified ratio of Si/Al had been applied, which was subsequentlysintered in an inert atmosphere at 1200° C., not any change in weightwas observed after correction for the change in the upward pressure uponincrease of the temperature.

EXAMPLE II

Application of ZSM-5 (MFI) Zeolite Crystallites on a Stainless SteelSubstrate.

In this case, the starting point was a layer of porous silicon dioxideprepared by applying silicone rubber to stainless steel anddecomposition of the silicone rubber layer at 773 K. As templatemolecule, the tetramethyl-ammonium from CFZ (Chemische FabriekZaltbommel) was used. Together with NaOH the tetrapropylammonium wasimpregnated in the pore volume of the porous layer. Subsequently, thezeolite was synthesized under hydrothermal conditions at 140° C.Photographs clearly showed that the resultant surface was homogeneouslycovered with zeolite crystallites.

What is claimed is:
 1. A method for applying a non-permeable, ceramiclayer of a thickness of not more than about 100 μm to a ceramic ormetallic body by applying a solution of one or more organometallicelastomers, optionally drying, and pyrolyzing to form a porous layer andsubsequently sintering at increased temperature to form a non-permeableceramic layer.
 2. A method according to claim 1, wherein the solution ofelastomers further contains aluminum and/or titanium ions.
 3. A methodaccording to claim 2, wherein the solution of elastomers furthercontains titanium dioxide.
 4. A method according to claim 3, wherein thesolution of elastomers further contains fluorine ions.
 5. A methodaccording to claim 4, wherein: the solution of elastomers furthercontains zirconium ions; the sintering is carried out in an inertatmosphere; the composition of the non-permeable ceramic layer to beobtained is adjusted by impregnating the porous layer with a solution ofdesired constituents; and substantially exclusively the pore volume ofthe porous layer is impregnated.
 6. A method according to claim 1,wherein the solution of elastomers further contains zirconium ions.
 7. Amethod according to claim 1, wherein the sintering is carried out in aninert atmosphere.
 8. A method according to claim 7, wherein: thecomposition of the non-permeable ceramic layer to be obtained isadjusted by introducing desired constitutents into the porous layer bydeposition-precipitation; priorly an oxide layer has been applied to thebody; the solution of elastomers further contains a catalytically activecompound; the porous layer, which further contains, if necessary,aluminum, is impregnated with constituents necessary for zeolitesynthesis, and that zeolite synthesis occurs by employing conditionsneeded therefor.
 9. A method according to claim 1, wherein thecomposition of the non-permeable ceramic layer to be obtained isadjusted by impregnating the porous layer with a solution of desiredconstituents.
 10. A method according to claim 1, wherein the compositionof the non-permeable ceramic layer to be obtained is adjusted byintroducing desired constituents into the porous layer bydeposition-precipitation.
 11. A method according to claim 1, whereinpriorly an oxide layer has been applied to the body.
 12. A methodaccording to claim 1, wherein the solution of elastomers furthercontains a catalytically active compound.
 13. A method according toclaim 1, wherein the porous layer, is impregnated with constituentsnecessary for zeolite synthesis, and that zeolite synthesis occurs byemploying conditions needed therefor.